1. Field of the Invention
The present invention relates to an apparatus for controlling a continuously variable transmission and to a method thereof. Particularly, the invention relates to technology for optimizing a rate of changing the speed of a continuously variable transmission.
2. Related Art of the Invention
A conventional method of controlling a rate of changing the speed of a continuously variable transmission has been disclosed in, for example, Japanese Unexamined Patent Publication No. 8-74958.
According to this method, the up-shift is divided into an up-shift in a state where the throttle opening degree remains nearly constant (hereinafter referred to as a first up-shift mode) and an up-shift accompanying a change in the throttle opening degree, and the up-shift control operation is carried out at different rates of changing the speed.
Even in the same up-shift operation accompanying a change in the throttle opening degree, however, it has been confirmed through experiment that a different rate of changing the speed is required depending upon the up-shift of when the throttle valve is fully closed (hereinafter referred to as a second up-shift mode) and the up-shift of when the throttle valve is closed nearly half (hereinafter referred to as a third up-shift mode).
That is, it is generally preferable that the rate of changing the speed is larger during the third up-shift mode than during the second up-shift mode. When the rate of changing the speed of during the third up-shift mode is set to a relatively small rate that meets the second up-shift mode, the rotational speed of the engine does not decrease as desired despite the throttle valve is closed and the driver feels it offensive. Conversely, when the rate of changing the speed of during the third up-shift mode is set to be relatively large that meets the first up-shift mode, the rotational speed of the engine decreases to an excess degree causing the driver to feel it offensive.
According to the prior art, however, the speed is changed at the same rate when the up-shift operation is effected accompanying a change in the throttle opening degree; i.e., the speed is not changed at a rate to meet the second up-shift mode or the third up-shift mode, and the driver feels it offensive when the speed is changed.
According to the prior art, furthermore, the speed is changed at the same rate during the down-shift operation; i.e., the transmission has not been so constructed as to discriminate the kind of down-shift operation to change the speed at a different rate. Even in the same down-shift operation, it has been confirmed through experiment that it is desired to change the speed at a different rate depending upon the down-shift to meet the demand for acceleration by opening the throttle valve (hereinafter referred to as first down-shift mode) and the down-shift that is effected when the vehicle speed has dropped (hereinafter referred to as second down-shift mode). According to the prior art, it is likely that the driver feels it offensive at the time of down-shift operation.